Hydrilla is the perfect aquatic weed

HAL SCHRAMM October 28, 2010 at 4:09pm

Hydrilla is a significant component of the habitat in several world-class bass fisheries - places like Fork, Guntersville, Seminole and Toledo Bend - but this invasive plant from Asia has choked waterways and blocked access to numerous excellent fisheries.

Well established in several states for more than three decades, hydrilla has finally found its way to Mississippi.

"Hydrilla makes these reservoirs the great bass fisheries they are," he said. "Hydrilla provides good habitat for all life stages of bass, and it provides easy-to-fish, visible cover that helps anglers find and catch bass."

But hydrilla is "well behaved" and rarely occupies more than 10 percent of the surface area of these giant East Texas reservoirs.

It's a totally different story in other lakes where hydrilla has colonized most, even all, of the lake and made access to fishing difficult or impossible. I've seen hydrilla cover almost the entire 13,000 acres of Orange Lake near Gainesville, Fla., a lake renown for excellent bass fishing. And Orange Lake is just one of many lakes shut down by excessive hydrilla

The biology of the beast

Hydrilla has physiological attributes that allow the plant to thrive with less light than needed by most native aquatic plants, enabling this invasive plant to colonize deeper water and outcompete native plants. New hydrilla plants can grow from old roots and buds (turions) on the plant stems and roots. Plant fragments can root and grow into new plants, which makes it so easy to spread hydrilla within a water body and to transport it to new waters.

Hydrilla's ability to dominate other aquatic plants and quickly spread has earned it the title of "the perfect aquatic weed." The operative word is "weed" - a plant growing where it isn't wanted.

The thick and leafy stem of hydrilla provides ample surface area for the growth of algae. The algae provides food, and the hydrilla plant provides substrate for high densities of aquatic insect larvae, grass shrimp and numerous other invertebrates that are eaten by sunfish and a variety of fish that are prey for largemouth bass. But native submerged plants provide the same benefits to the fishery.

Hydrilla also provides cover for the young of many fish including sunfish and bass. Clearly, hydrilla can increase the abundance and growth of sunfish and bass, but so can native submersed plants.

Too much hydrilla can impair fisheries. A Florida study found sunfish condition declined when hydrilla exceeded 40 percent of the lake surface area. Other fishery studies have suggested that submersed vegetation coverage up to 30 percent benefits fisheries. The important point is that native submersed plants provide the same fishery benefits as hydrilla without the risk of expanding to nuisance levels.

Extensive growth of hydrilla can work against fish production in several ways. Dense hydrilla can provide good cover for forage fishes, but the jungle of stems and leaves impairs forage detection and capture by predators like largemouth bass. Expansive monocultures of hydrilla offer relatively little edge where bass can be effective predators. Forage may be abundant, but it won't grow bass if the bass can't find it or catch it.

Unlike native submersed plants, hydrilla continues to grow when the stems reach the surface. This "topped-out" hydrilla can form a dense surface mat. With little light passing through, aquatic vegetation beneath the mat dies out. The hydrilla continues to survive, but only leafless stems remain. The abundant surface area that supported algae and aquatic invertebrates is now gone, as is the rich food supply for sunfish and forage fish. It's hard to sustain quality fisheries with an unstable food supply.

Hydrilla is established in Ross Barnett, several of the lakes on the Tenn-Tom Waterway, Pickwick Lake and golf course ponds in Harrison County. Hydrilla has been found at Wall Doxey State Park Lake, but early detection and eradication of only a few plants may have prevented the establishment and spread.

Management dilemma

Managing hydrilla in public waters is difficult to say the least. Every public water is a multi-use resource, and every system is different. Ross Barnett Reservoir has ample native aquatic plants, and the fertile, shallow reservoir is ripe for hydrilla colonizing vast areas of Mississippi's largest reservoir.

Mississippi Department of Wildlife, Fisheries and Parks fishery biologist Larry Bull was emphatic and to the point: "We have a great fishery without hydrilla. The ecologically and economically smart management strategy in Barnett is to eradicate the invader before it spreads."

At present, the hydrilla is being controlled with aquatic herbicides where hydrilla impairs access. This is an annual and expensive solution - chemical control of hydrilla can cost more than $1,000 per acre per year.

I surmise that Pickwick may become a management challenge very similar to Lake Guntersville, Ala., where it took years for all stakeholders to agree on aquatic plant-management goals and strategies, and now people are wondering who is going to pick up the pricey tab.

The coverage of hydrilla in Pickwick has steadily increased over the last five years. By September, broad, continuous mats of topped-out hydrilla grew on most shallow flats throughout the middle third of the lake. Pickwick has limited shallow water, and hydrilla will colonize only a relatively small area of the lake. Unfortunately, it is already growing where it is interfering with lakeside property owners' use of the lake.

Whatever hydrilla management solutions are enacted on the Tenn-Tom or Pickwick will cost money - money that could be better spent enhancing fisheries. One very proactive approach would be to invest in efforts to establish native aquatic plants in systems where they are lacking. Fisheries will usually benefit, and the established native plants may reduce the likelihood of hydrilla becoming established.

While fisheries and water-management agencies are monitoring and managing hydrilla, do your part as a conscientious angler to not spread hydrilla and other unwanted plants in our waters by removing all plant fragments from your boat and trailer before leaving the ramp.